The question is whether MP3tag still supports Windows XP ...
I remember that when I used MP3tag and XP that some system generated text boxes could not cope with special characters and showed squares instead.

Mike, it's not quite the same complaint. I think mine is rather different. In the other post you refer to, the installer has guessed the Thai locale correctly, but the font appears not to support it.

In my case, however, the installer fails to detect Estonian, and proceeds with some arbitrary language. I assume that Estonian is not included in the list of supported languages of this installer. But then it should either use English as the default (or German, it's up to the author) or offer the user to choose the preferred language.

This is not the question of this topic. According to the download page Windows XP SP3 is officially supported. If you notice any bugs on such a system which does not appear elsewhere, please report it in a separate topic.

In my case, however, the installer fails to detect Estonian, and proceeds with some arbitrary language. I assume that Estonian is not included in the list of supported languages of this installer. But then it should either use English as the default (or German, it's up to the author) or offer the user to choose the preferred language.

I've looked into this bug more closely and now it appears that Mike is correct: in fact, this is the same problem as mentioned in the referred post.

I've noticed that the text itself appears exactly the same in both cases (judging by the number of letters, repetitions and double-letters in the same locations), only character sets are different. After some experimenting with Google Translate and character encodings, I think the original text is in Arabic. (Because the last sentence should read "Click Next to continue.")

This means that Thai language was also not detected/supported by the installer. As the result in both cases, installer chose Arabic (perhaps because it is the first language in the list where language names are sorted alphabetically).

It seems like the original Arabic text the installer is trying to display is written in the Windows-1256 (Arabic) code page. Perhaps this would be the default code page on Arabic Windows PCs. However, because in my PC the default is the Windows-1257 (Baltic) code page, I see the gibberish pictured above. And the other customer (Illuminator, in the referred post) has the TIS-620 (Thai) character encoding set as default on their PC. So the characters appear different (Thai script), but in fact the text is completely unreadable because it is originally in Arabic.

To sum up: Arabic appears to be the language always chosen when the installer doesn't support the user's language.